In the case of digitizing a signal of a moving picture to record and transmit digital data thus obtained, its data quantity becomes extremely large. For this reason, coding (compression) is implemented to that data. As a representative coding system, there is so called MPEG (Moving Picture Expert Group) system. This MPEG is general name of the moving picture coding system which has progressed in WG (Working Group) 11 of SC (Sub Committee) 29 of JTC (Joint Technical Committee) 1 of ISO (International Standardization Organization) and IEC (International Electrotechnical Commission).
Moreover, as a widely known format in the case of handling a picture signal by a digital signal, there is Recommendation 601 (Rec. 601) of so called CCIR (Comite Consultatif Internationale des Radio-communications (International Radio Consultative Committee)). This Rec. 601 is the worldwide standardized format of digital picture based on the component (so called 4:2:2 component) coding system.
In the MPEG, the above-mentioend Rec. 601 format is used as input/output picture format. In the NTSC zone of the scanning line 525 lines/30 Hz, an input/output picture format as shown in FIG. 11, for example, has been most popularly used. For example, in MPEG2 (MPEG phase-2), picture of picture frame of frame of 720 pixels.times.480 lines of Rec. 601 format is handled. On the other hand, in MPEG1 (MPEG phase-1), picture of picture frame of frame (SIF) of 360 pixels.times.240 lines obtained by thinning pixels so that resolutions in the longitudinal and lateral directions are respectively equal to one half thereof is handled.
A technique to allow the transmission format of picture to have a relationship such that the ratio between length and breadth of picture frame is expressed as 1:2 in dependency upon difference of resolution is convenient in a hierarchical manner. This is related to the following two reasons.
First reason is that hardware configuration of thinning filter and/or interpolating filter having ratio of 1:2 are simpler than those of filters having other ratios.
Second reason is that, in the case of constituting picture coding/decoding apparatuses, it is possible to process picture data by four image LSIs disposed in parallel for picture having resolution which is one half of picture to be processed.
An example of filter coefficients of the thinning filter is shown in FIG. 12.
Moreover, since there is employed a scheme to handle picture in a manner divided into small blocks called macro blocks of 16 pixels.times.16 lines, it is preferable that the horizontal and vertical sizes of picture are multiple of 16 when coding efficiency is taken into consideration. From a viewpoint of this, picture frame size of 720 (=16.times.45) pixels.times.480 (16.times.30) lines is preferable size. For reasons described above, in the NTSC zone, picture of picture frame of frame of 720 pixels.times.480 lines of Rec. 601 format has been widely handled.
On the other hand, CCIR recommended broadcasting stations (television stations) that picture signals of 483 lines or more (about 483-486 lines) must be transmitted in the standard television broadcasting in the NTSC zone. At this time, if picture coding is carried out by MPEG as in the prior art, it is required to increase the number of macro blocks by one column in a longitudinal direction to carry out processing of 720 pixels.times.496 (480+16) lines. This results in requirement of increase in the coding processing ability of about 3.5% as compared to the case where frame of 720 pixels.times.480 lines is handled.
This is inconvenient for a system only having a decoding apparatus which can merely handle so far as picture of picture frame of frame of 720 pixels.times.480 lines. For example, when the recording system of the digital video disc and the broadcasting system of the digital television are the same MPEG, it is considered that there results the merit that it is sufficient to have only either one of decoding apparatuses. However, in the case where decoding apparatus of video disc player has only processing ability of picture frame of 720 pixels.times.480 lines, it is unable to receive picture signals in digital television broadcasting by that decoding apparatus. This gives rises to extremely waste from a viewpoint of effective utilization of hardware resource.
Meanwhile, CCIR only recommended that picture signals of 483 lines or more (about 483-486 lines) in the standard television broadcasting in the NTSC zone must be transmitted, but whether or not the receiving side decodes picture information of 480 lines or more is arbitrary. For this reason, in the case where the receiving side is a decoding apparatus having only processing ability of picture frame of 720 pixels.times.480 lines, if it is possible to decode only such picture frame, this is very desirable.
To meet the above-described requirement, it becomes necessary to realize a moving picture coding technology required for providing moving picture coded information (broadcasting) having flexibility and a decoding method therefor such that in the case where the receiving side is a decoding apparatus having processing ability of picture frame of 483 lines or more, received all picture signals can be reproduced, while in the case where the receiving side is a decoding apparatus having only processing ability of picture frame of 480 lines, only the decordable picture portion can be reproduced.
To realize this, it is necessary to solve problems described below.
First, such a task must be accomplished to permit discrimination as to whether or not only picture information of reproducible picture frame is taken out from received coded information.
Secondly, since in the coding method using motion compensating prediction between pictures such as MPEG, etc., pictures have correlation therebetween in a time base direction, in the case where reference is made to the outside (line portions indicated by 16 lines in the figure, for example) of the reproducible picture frame by motion compensation as shown in FIG. 13, motion compensation is unable to be carried out. As a result, decoding cannot be made. Accordingly, it is necessary to take any measure for this problem.
Namely, the case where coded information of 496 lines is received by decoder only having processing ability of picture frame of 480 lines is taken as an example in FIG. 13. In the case of the example of FIG. 13, motion compensation in directions indicated by arrows a, b in the figure from within the picture frame of 480 lines of past picture or future picture to current picture can be made, but motion compensation in directions indicated by arrows c, d in the figure from within the picture frame beyond 480 lines of past picture or future picture (line portions indicated as 16 lines in the figure) to current picture cannot be carried out.